1. Field of the Invention
The present invention relates to an optical wavemeter employing a length measuring machine.
2. Description of the Related Art
FIG. 3 is a schematic diagram of an optical wavemeter of a prior art. In FIG. 3, denoted at 1 is a light source to be measured, 3 is a beam splitter, 4 is a fixed mirror, 5 is a movable mirror, 6 is a length measuring machine, 9 is a light receiver, 10 is a converter, 14 is a calculator, 15 is a display, 16 is a movable mirror driving portion, 17 is a linear moving mechanism, 21 is a position detecting portion, 22 is a distance counting portion and 23 is an interfering light counting portion.
The length measuring machine 6 illustrated in FIG. 3 comprises a scale 61 and a sensor 62. The movable mirror driving portion 16 comprises a motor portion 16A, pulleys 16B and 16C, a belt 16D and limit switches 16E and 16F. The belt 16D is made of, for example, a rubber belt.
A beam 1A having an unknown wavelength which is emitted by the light source 1 to be measured is branched by the beam splitter 3 into two beams, i.e., a passing beam 1B and a reflected beam 1C. The reflected beam 1C is further reflected by the fixed mirror 4, e.g., a corner-cube prism, and passes through the beam splitter 3 to be incident to the light receiver 9. Whereas the passing beam 1B is reflected by the movable mirror 5, e.g., the corner-cube prism and further by the beam splitter 3 to be incident to the light receiver 9.
At that time, the passing beam 1B and the reflected beam 1C which are incident to the light receiver 9 interfere with each other to form composite light 1D in the light receiver 9, which supplies an electric signal 9A corresponding to the strength of interfering light to the converter 10. The converter 10 converts the electric signal 9A from the light receiver 9 into pulses to supply the same to an interfering light counting portion 23.
The movable mirror 5 is fixed to the belt 16D stretched between the pulleys 16B and 16C of the movable mirror driving portion 16. When a motor rotates in the motor portion 16A, the pulley 16C which is fixed to the motor is rotated to move the movable mirror 5 fixed to the belt 16D on the linear moving mechanism 17 in the direction of an optical axis. As the movable mirror 5 moves in the direction of the optical axis, the electric signal 9A issued by the light receiver 9 becomes an electric signal which corresponds to the periodically repeating variation of light intensity due to interference. The wavelength of the electric signal 9A is the same as that of the beam 1A to be measured.
The length measuring machine 6 supplies a pulse signal 6A to the position detecting portion 21 and the distance counting portion 22 every time the movable mirror 5 moves by the distance resolution of the sensor 62. The sensor 62 supplies an origin signal 6B to the position detecting portion 21 when the movable mirror 5 passes the center of the scale 61.
The position detecting portion 21 counts the number of pulses in the pulse signal 6A being triggered by the origin signal 6B supplied from the sensor 62 of the length measuring machine 6 and when the movable mirror 5 moves by an arbitrary distance, that is, the number of pulses in the pulse signal 6A reaches an arbitrary number, the position detecting portion 21 outputs a position signal 21A to stop.
When the interfering light counting portion 23 receives the position signal 21A from the position detecting portion 21, it starts counting the number of pulses in a signal supplied by the converter 10 and stops counting when it receives the position signal 21A from the position detecting portion 21 again to output a counting result K to the calculator 14.
When the distance counting portion 22 receives the position signal 21A from the position detecting portion 21, it starts counting pulses in the pulse signal 6A from the sensor 62 of the length measuring machine 6 and when it receives the position signal 21A from the position detecting portion 21 again, stops counting to output a counting result N to the calculator 14.
The calculator 14 performs calculation by substituting the counting result N supplied thereto from the distance counting portion 22 in Equation: L=N.times.(resolution of the length measuring machine), wherein L is the moving distance of the movable mirror 5, and further substituting the moving distance L and the number of pulses K supplied thereto from the interfering light counting portion 23 in Equation: .lambda.=2L/K to obtain .pi., i.e., the wavelength of the light to be measured so as to output the same to the display 15, which displays the wavelength data of the light to be measured thereon.
The motor portion 16A reverses the rotating direction of the motor therein when the movable mirror 5 contacts with either of the limit switches 16E and 16F. A plurality of data can be obtained by repeatedly moving the movable mirror and the wavelength can be measured more accurately by averaging the data.
FIG. 4 shows the configurations of the movable mirror 5 and the length measuring machine 6 in the optical wavemeter in FIG. 3. In FIG. 4, the movable mirror 5 can move horizontally on the linear moving mechanism 17. The moving distance of the movable mirror 5 is measured by the scale 61 and is detected by the sensor 62, The scale 61 is provided with an origin at the central portion of the scale 61.
In order to improve in measuring accuracy the optical wavemeter having a structure illustrated in FIG. 3, it is necessary to improve the measuring accuracy of the length measuring machine 6 and lengthen the measuring length. For this purpose, the length measuring machine 6 in FIG. 4 employs a scale 61 having an origin and light having a known wavelength such as frequency-stabilized laser is incident to the optical wavemeter for distance calibration between the origin of the scale 61 of the length measuring machine 6 and an arbitrary moving limit of the movable mirror 5.
However, in FIG. 4 for example, the distance calibration of the scale 61 can be performed only in either of a range between A and the center thereof and that between B and the center thereof though the scale 61 is effective for measurement in a range between A and B about the origin thereof. As a result, effective calibration range is reduced to a half of full scale so as to reduce the accuracy of the wavemeter.
Although it is possible to make the measuring distance large by moving the movable mirror 5 from the origin of the scale 61 to A in advance and then performing measurement between A and B after performing calibration between A and the origin of the scale 61 and between B and the origin of the scale 61, the movable mirror 5 is moved by the motor in the motor portion 16A by way of the belt 16D of the movable mirror driving portion 16 as illustrated in FIG. 3, so that when the movable mirror 5 moves from the center of the scale 61 to A and stops there, the movable mirror 5 does not always stop at the same position due to the inertia of the motor in the motor portion 16A or the elasticity of the belt 16D and consequently the calibration is not valid for correct measurement of wavelength.